This invention relates generally to wireless data transmission systems and in particular to a technique for using directional antennas in such systems.
In corporate enterprises, wireless Local Area Networks (LANs) are usually implemented as a final link between existing wired networks and a group of client computers. Today's business environment is characterized by an increasingly mobile work force, who spend much of their time working in teams that cross functional, organizational and geographic boundaries. Often their most productive times occur in meetings that take place away from their desks. Users of portable computing equipment therefore need access to their data files through networks that reach far beyond their personal desktops. Wireless LANs fit well into this environment, providing much needed freedom in network access to mobile workers. Such networks provide access to information from anywhere within an enterprise, such as from a conference room, cafeteria, or even a remote branch office. Wireless LAN connectivity affords access to the full resources and services of a corporate network across a building or campus setting. As such, they are on the verge of becoming a mainstream solution for a broad range of business applications.
One critical issue affecting the effectiveness of wireless LAN deployment has been the historically limited throughput available with such equipment. The 2 Mega bits per second (Mbps) data rate specified by the original Institute of Electrical and Electronics Engineers (IEEE) wireless LAN standard 802.11, dated 1997, is now considered to be too slow to support most business requirements. Recognizing the need to support additional higher data rate transmissions, the IEEE recently ratified an 802.11b standard that specifies data transmission speeds of up to 11 Mbps. With the 802.11b standard, wireless LANs are expected to be able to achieve throughput comparable to the legacy wired Ethernet infrastructure. Emerging wireless networking systems that promise to provide comparable data speeds include Home RF, BlueTooth, and third generation digital cellular telephone systems.
In these peer-to-peer networks, the individual computer nodes operate in a same frequency communication network. That is, these systems utilize signal modulation schemes such as Code Division Multiple Access (CDMA) wherein a number of end nodes are actually transmitting on a same radio frequency carrier at the same time. Such systems may experience significant quality degradation due to the interference of equipment that is not participating in system communication processes. For example, wireless LAN systems typically operate in unlicensed radio frequency bands. Other types of radio equipment operate in these bands, equipment that is not required to operate in accordance with the promulgated LAN standards, and therefore, cannot be controlled. These transmissions from such non-system nodes can significantly reduce the performance of a wireless LAN. As data rates increase, susceptibility to such interference also increases accordingly.
Various other problems are inherent in wireless communication systems. One such problem is the so-called multipath fading problem whereby a radio frequency signal transmitted from a sender (either a base station or another mobile subscriber unit) may encounter interference enroute to an intended receiver. The signal may, for example, be reflected from objects such as buildings that are not in a direct path of transmission but then are redirected as a reflected version of the original signal to the receiver. In such instances, the receiver actually receives two versions of the same radio signal: the original version and a reflected version. Since each received signal is at the same frequency but out of phase with the other due the longer transmission path for the reflected signal, the original and reflected signals may tend to cancel each other out. This results in dropouts or fading of the received signal.
Radio units that employ single element antennas are highly susceptible to such multipath fading. A single element antenna has no way of determining a direction from which a transmitted signal is sent and cannot be tuned or attenuated to more accurately detect or receive a signal in any particular direction operating by itself. It is known that directional antennas can therefore alleviate both the multipath fading and similar problems to some extent.